Active Low or Active High

On 16 Oct 2007 23:46:52 -0700, Harry wrote:

>Hi all,
>
>We all know that microprocessors/microcontrollers have active low &
>active high signals.Is there any specific advantage for active low
>signals?Can't it be all active high or active low?

The active low was very common on old bipolar logic families, such as
RTL, DTL or TTL.

Since in those days it was not practical to make both NPN and PNP
transistors on the same chip, so only NPN transistors were used. The
output stage was a Common emitter NPN stage and it was easy to drive
current into the base of the NPN, putting the transistor into
saturation, with a large current flowing from collector to emitter
with a low voltage drop, thus the output is low, when current flows
into the base of the output transistor.

It would be very hard to make an active high output stage, since an
emitter follower transistor with the collector at Vcc and the output
at the emitter. You would actually need an additional supply voltage
higher than Vcc in order to be able to drive sufficient current into
the base of the NPN transistor in order to get the output voltage even
close to Vcc.

Of course ECL used NPN emitter followers with negative Vee at -5.2 V,
but that was an oddity.

Paul

Martin Griffith wrote:
>
>Vladimir Vassilevsky wrote:
>
>>Harry wrote:
>>
>>> We all know that microprocessors/microcontrollers have active low &
>>> active high signals.Is there any specific advantage for active low
>>> signals? Can't it be all active high or active low?
>>
>>It is the deep historical and religious question. It has to do with the
>>fact that for the ancient 74xx series the outputs normaly act as drains,
>>and the inputs act as sources.
>
>I always thought that it went way back, in relay circuits. I have an
>idea that it was so you could not easily short the supply to ground
>and pop fuses.

The original relay circuits were mostly used in positive ground
telephone circuits, so it is the high condition that is less likely
to short out and blow fuses.

It's an interesting story as to how it came to be that telephones
are mostly positive ground and modern cars are mostly negative
ground. When Michael Faraday did his pioneering work on electrolysis
he determined that what we now call the positive electrode (anode)
lost material and what we now call the negative electrode (cathode)
gained material. So he called the electrode that was putting material
into the liquid positive and the electrode that was taking material
out of the liquid negative. He had no way of knowing that there were
electrons moving from negative to positive and ions moving from
positive to negative.

Because whichever part of the system is negative gains metal and
whichever part of the system is positive loses metal, the designers
of the telephone system made the ground positive, because in a
telephone system, ground stakes are a lot easier to replace than
wires are. In automobiles, body and frame parts are harder to
replace than wires are, so the automotive designers eventually
settled on negative ground.

Telegraphs had a different system: a battery on each end. This
resulted in negative ground at one end and positive ground at
the other end, and ground halfway between positive and negative
somewhere in between.


--
Guy Macon

Martin Griffith wrote:
> Vassilevsky wrote:
>> Harry wrote:
>>
>>> We all know that microprocessors/microcontrollers have active
>>> low & active high signals.Is there any specific advantage for
>>> active low signals?Can't it be all active high or active low?
>>
>> It is the deep historical and religious question. It has to do
>> with the fact that for the ancient 74xx series the outputs
>> normaly act as drains, and the inputs act as sources.
>
> I always thought that it went way back, in relay circuits. I
> have an idea that it was so you could not easily short the
> supply to ground and pop fuses.

Actually the immediate predecessor (to NPN DTL gates) were PNP DTL
gates, which swang between a negative voltage and ground. However,
the ground level was still the power direction. They went out
about 50 years ago, but reappeared in early PMOS designs.

--
Chuck F (cbfalconer at maineline dot net)
Available for consulting/temporary embedded and systems.




--
Posted via a free Usenet account from http://www.teranews.com